The invention relates to an optical scanning apparatus for flat surfaces comprising a laser light source. A periodic light deflecting means such as a mirror wheel is illuminated by a laser beam. A telecentric optical scanning system which directs the laser beam, is periodically swept over a specific angular range by the light deflecting means. The light deflecting means is in the form of a scanning beam which is displaced parallel to itself, at an angle of incidence (other than zero) towards the flat surface. There, a light bead arises which periodically scans the flat surface along a scanning line. A light receiving means is provided for the light reflected from the region of the scanning line on the surface.
Such optical scanning apparatuses (for example, as described in German Offenlegungsschrift No. 35 04 019 corresponding to U.S. Pat. No. 4,775,238) are used for seeking faults at regular or specularly reflecting surfaces. Here it is necessary in order to detect faults with a small beam deviation to render the fluttering of the flat surface harmless during the signal recording, with such fluttering occurring in particular when the flat surface is the surface of a moving material, for example a web of sheet metal in a rolling mill.
However, it sometimes happens, with a specularly reflecting material web, that waves or corrugations extending perpendicular to the scanning line, are to be detected with the light receiving apparatus in addition to individual surface faults, i.e. waves or corrugations of which the crests and troughs also extend parallel to the scanning line.
An apparatus for amplifying the dependence of the angular changes of an optical emergent beam on the angular changes of the associated incident beam is known from German Auslegeschrift No. 20 42 508, corresponding to U.S. Pat. No. 3,771,850. The known apparatus has a first pivotable mirror which is arranged with its pivot axis in the front focal point of a first lens. A second lens is provided on the side of the first lens remote from the pivotable mirror with the front focal point of the second lens coinciding with the rear focal point of the first lens. A second mirror is arranged in the rear focal point of the second lens and is perpendicular to the common optical axis of the two lenses.
A parallel light beam emerging from the fixed front focal point of the first lens is thus imaged by the first lens depending on the angle of incidence along a line in the rear focal plane. The second lens then forms an image of each point of the common focal plane at its inverse point via the second plane mirror. The reflected parallel light beam which leaves the first lens then reaches the front focal point of the first lens at the inverse angle. If the first pivotable mirror is not perpendicular to the common optical axis of the two lenses then the angle of incidence of the parallel light beam on the first mirror can be increased in this manner.